Twenty percent of our catalog, or 424 galaxies, have no counterpart in the HST source catalogs. While a few of these galaxies are clearly visible in the HST images and were just missed by the SExtractor source de-blending, the vast majority are too faint with respect to the HST limiting magnitude, which is between 29 and 30 AB magnitude depending on the filter. These new sources are high equivalent-width \(\mathrm {Ly}\alpha \)  emitters that are detected by MUSE thanks to their bright and asymmetric broad Ly line. As expected, the fraction of HST-undetected \(\mathrm {Ly}\alpha \)  emitters is a function of depth, with a sharp increase from 18% in the 10-hour depth MOSAIC to 49% in the 140-hour depth MXDF.
These figures confirm the existence of a large population of faint star-forming \(\mathrm {Ly}\alpha \)  emitters revealed in MUSE deep fields [1]}, [2]}. As shown by [3]}, [4]}, these galaxies are on average very faint (\(\rm M_{UV} \approx -15\) ) star-forming galaxies with a high ionizing photon production efficiency.
